Light emitting devices or diodes (LEDs) are based on a forward biased p-n semiconductor junction. LEDs have recently reached high brightness levels that have allowed them to enter into new solid state lighting applications as well as replacements for high brightness light sources such as light engines for projectors and automotive car headlights. These markets have also been enabled by the economical gains achieved through the high efficiencies of LEDs, as well as reliability, long lifetime and environmental benefits. These gains have been partly achieved by use of LEDs that are capable of being driven at high currents and hence produce high luminous outputs while still maintaining high wall plug efficiencies.
Solid state lighting applications require that LEDs exceed efficiencies currently achievable by incandescent and fluorescent lighting technologies. Currently, one of the preferred routes for the generation of White light from an LED module is by use of a single colour LED (such as a blue LED) and a wavelength converting element (such as a yellow phosphor). Wavelength converting elements (WCE) typically comprise of a yellow phosphor mixed in an encapsulant and dispensed at the correct composition on top of a blue LED chip to generate a white colour of the desired colour temperature. By modifying the fill fraction composition or % weight the white light colour may be tuned. The ability to provide white light across a large chromaticity space is advantageous for different lighting applications. However, due to manufacturing inaccuracies associated with variation in LED emission wavelength, LED emission bandwidth, variation in WCE % weight and WCE composition different LED modules will exhibit white light emission characteristics with different chromaticity values. This is undesirable as sorting and binning of LED modules post manufacture is required. Additionally, the spectrum of the total emitted light (TEL) arising from Blue and yellow emission typically provides a low to medium colour rendering index (CRI) in the range 60-80.
U.S. Pat. No. 6,788,011 describes the mixing of Red, 102, Green, 103, and Blue, 104, primary colour semiconductor LEDs to provide white colour light as shown in FIG. 1a. In order to achieve the desired light intensity as well as the colour chromaticity on a CIE diagram (a standard colour space created by the International Commission on Illumination), a control system is programmed with predefined LED driver power values for each individual LED colour. The light emission spectra intensity, 106, plotted against wavelength (as shown along 105) for each LED namely the Red LED, 109, Green, 108 and Blue, 107 are shown on the insert in FIG. 1a. The individual LEDs are assembled in a housing or board, 101. This LED lighting system suffers from several drawbacks, as detailed below.
Firstly, the LED lighting system suffers from poor Colour Rendering Index (CRI) typically around 27-30 because of the individual narrow Red, Green and Blue wavelengths (approximately 10-25 nm bandwidth wavelength emission) providing poor representation of the complete visible spectrum of light, which is typically experienced from incandescent bulb illumination or blackbody radiation, 120, as shown in the insert in FIG. 1a. As a reference, the CRI for a blackbody radiation is 100 and the value ranging between 0 and 100 defines how accurately light will portray colours relative to a blackbody source at the same nominal colour temperature.
Secondly, due to the different LED semiconductor material systems required to generate Red (typically InGaAlP) and Blue or Green (InGaAlN) wavelengths, the relative light intensity, voltages, lifetime and junction temperature may dramatically vary from one LED to another. It is also important to note that when the LED junction temperature is increased, the relative light output from a light emitting device comprising of a InGaAlP material system is degraded at a greater rate than a light emitting device comprising of a InGaN material system and hence all these factors will adversely affect the overall light intensity, colour chromaticity point and colour quality with lifetime and temperature giving rise to an LED lighting system that is unstable and not useable. This is typically very difficult to monitor without the addition of feedback control systems.
In U.S. Pat. No. 7,213,940B1 another colour control system is proposed, whereby a first semiconductor LED with a first lumiphor is provided to generate white light. In order to improve the CRI, a second semiconductor LED having a different emission wavelength is introduced into the optical mixing. This system provides much improved Colour Rendering Index (CRI) of around 80-92 due to the broader emission achieved by the first LED and lumiphor. The introduction of the second semiconductor LED with Red emission wavelengths has a limited emission bandwidth and hence is restricted in the amount that the CRI can be increased. Secondly, the external efficiency of state of the art commercial red emitting semiconductor materials such as InGaAlP is typically 30%, which is much lower than that of GaN based blue emitting semiconductor LED systems (state of the art commercial LED external efficiency at 45%). Additionally, similar lifetime degradation problems compared to LED lighting devices in U.S. Pat. No. 6,788,011 are also experienced with the second semiconductor degrading at a different rate to the first LED.
In published U.S. Patent Application No. 2008/0048193 A1 a white LED module including a further circuit board is described. The LED module cross sectional schematic is shown in FIG. 1c. In one example of the invention a Green semiconductor LED, 103 and a Blue semiconductor LED, 104 are placed on circuit board 101. A Red phosphor, 112, is disposed over 103 and 104 to provide a total emitted white light intensity, 106, against wavelength 105. The total emitted white light has a broad red phosphor emission, 109, and narrow blue, 107, as well as a narrow green emission, 108, from the semiconductor LED die. The white light generated from the LED module suffers from a poor CRI (expected to be approximately 50-60) due to the narrow light emissions in the Blue and Green wavelength regions. Additionally, the intensity of the Green light, 113, is dramatically attenuated, 108. The total green light initially emitted from the LED die 103 is shown as a dotted line, while following the propagation through the Red phosphor 112 the final transmitted green light is shown as the solid line 108. The attenuated green light dramatically affects the total efficiency of the LED module. It is important to note that this applies across all wavelengths and not specifically for Red phosphors only.
As will be appreciated by those skilled in the art, there is currently a need for a LED module that combines the known benefits of low cost LED modules with the functionality of tunable colour chromaticity. It would be desirable to provide a module having uniform chromaticity properties, and which also displays good CRI and stable light intensity with a long lifetime.